Update OpenGL Viewport when Window resizes (#2915)

* Update OpenGL Viewport

* Comment update

* Make glViewport static, fix check, add exceptions

* Add warning

---------

Co-authored-by: Dan Lawrence <danintheshed@gmail.com>

Author: damusss

src_c/window.c

@@ -3,6 +3,7 @@
 #include "pygame.h"
 
 #include "pgcompat.h"
+#include "pgopengl.h"
 
 #include "doc/sdl2_video_doc.h"
 #include "doc/window_doc.h"
@@ -85,6 +86,7 @@ pg_display_resource(char *filename)
 }
 
 static PyTypeObject pgWindow_Type;
+static GL_glViewport_Func p_glViewport = NULL;
 
 #define pgWindow_Check(x) \
     (PyObject_IsInstance((x), (PyObject *)&pgWindow_Type))
@@ -201,7 +203,24 @@ window_flip(pgWindowObject *self, PyObject *_null)
     Py_RETURN_NONE;
 }
 
-// Callback function for surface auto resize
+/* Exception already set */
+static int
+_window_opengl_set_viewport(SDL_Window *window, SDL_GLContext context,
+                            int wnew, int hnew)
+{
+    if (SDL_GL_MakeCurrent(window, context) < 0) {
+        PyErr_SetString(pgExc_SDLError, SDL_GetError());
+        return -1;
+    }
+    if (p_glViewport == NULL) {
+        PyErr_SetString(pgExc_SDLError, "glViewport function is unavailable");
+        return -1;
+    }
+    p_glViewport(0, 0, wnew, hnew);
+    return 0;
+}
+
+// Callback function for surface auto resize or OpenGL viewport update
 static int SDLCALL
 _resize_event_watch(void *userdata, SDL_Event *event)
 {
@@ -222,6 +241,15 @@ _resize_event_watch(void *userdata, SDL_Event *event)
         return 0;
     }
 
+    if (event_window_pg->context != NULL) {
+        if (_window_opengl_set_viewport(event_window, event_window_pg->context,
+                                        event->window.data1,
+                                        event->window.data2) < 0) {
+            return PyErr_WarnEx(PyExc_RuntimeWarning,
+                                "Failed to set OpenGL viewport", 0);
+        }
+    }
+
     if (!event_window_pg->surf)
         return 0;
 
@@ -578,6 +606,13 @@ window_set_size(pgWindowObject *self, PyObject *arg, void *v)
          * relying on the event callback */
         self->surf->surf = SDL_GetWindowSurface(self->_win);
     }
+    if (self->context != NULL) {
+        /* Update the OpenGL viewport immediately instead of relying on the
+         * event callback */
+        if (_window_opengl_set_viewport(self->_win, self->context, w, h) < 0) {
+            return -1;
+        }
+    }
 
     return 0;
 }
@@ -966,6 +1001,11 @@ window_init(pgWindowObject *self, PyObject *args, PyObject *kwargs)
             PyErr_SetString(pgExc_SDLError, SDL_GetError());
             return -1;
         }
+        /* As stated in the 'Remarks' of the docs
+         * (https://wiki.libsdl.org/SDL2/SDL_GL_GetProcAddress) on Windows
+         * SDL_GL_GetProcAddress is only valid after an OpenGL context has been
+         * created */
+        p_glViewport = (GL_glViewport_Func)SDL_GL_GetProcAddress("glViewport");
         self->context = context;
     }
     else {